In the past fusing of thermal spray coatings on waterwalls has been accomplished by using a range of combustible gases (natural gas, propane, acetylene, propylene, etc.) and oxygen, and torch devices. The heat applied by the torch heats the coating and tube to the liquidus temperature of the coating, thus allowing the coating to "braze" onto the prepared surface of the waterwall to form a metallurgical bond. However, this process has a number of drawbacks. First it is difficult to fuse the membrane portion of the waterwall without overheating, burning, or melting off the coating from the sidewalls or crowns of the tubes. Another problem with torch fusing is the requirement for high amounts of heat (BTUs) to heat the waterwall to a sufficiently high temperature to allow the coating to reach its liquidus temperature. The high thermal conductivity and thermal mass of the waterwall pulls heat away from the coating very quickly. Only after the waterwall section has been heated throughout the tube thickness will the coating begin to fuse. As a result, the practice is to heat sufficiently to fuse the crowns of the tubes only, leaving the membranes unfused. This high heat input leads to warpage of the tubes and waterwall, as well as introducing potential microstructural changes into the tubes, which may have adverse effects on waterwall performance or usage. Attempts to cool tubes with water passing through the tubes were shown to pull the heat out of the system too quickly, as the torches provided too few BTUs to overcome the conduction of heat away from the area of concern. Thirdly, torch fusing techniques are also very time-consuming and difficult to control with any consistency.
Induction heating has been used to heat and fuse thermal spray coatings on individual straight tubes and rods in the past, but no record of using induction heat fusing on a complicated shape such as a waterwall panel is known. Previous efforts have focused on relatively low frequency (&lt;10 kHz) usage of induction heating techniques. The result has been that heat penetration into the base material is greater, thereby increasing the possibility of overheating and warping the article. In the time frame of the prior work, higher frequency equipment, and hand-held transformers were not utilized for this application.
The invention comprises a method of providing a continuous fused coating over waterwall panels by means of a specially designed induction coil, which provides uniform heating to both waterwall tube and membrane; and the invention also relates to coil itself. The invention is particularly useful for fusing conventional formulations of self-fusing alloy thermal spray coatings, such as nickel based alloys, which may include other components such as boron and/or silicon, chrome, molybdenum, iron, titanium, chrome carbides, tungsten carbides, and others; however the invention is also applicable for use with vitreous ceramic coatings, such as compositions of low melting point frits with an inorganic binder.
According to one aspect of the present invention a method of fusing a self-fusing alloy thermal spray coating or a vitreous ceramic coating on a waterwall panel having a plurality of tubes interconnected by a plurality of membranes is provided. The method comprises the steps of: (a) Heating at least some portions of at least one membrane and adjacent tubes of the waterwall panel, by induction, to a liquidus temperature of a self-fusing alloy thermal spray coating or a vitreous ceramic coating without significant warpage or adverse change in the microstructure of the material forming the panel. And, (b) applying a self-fusing alloy thermal spray coating or a vitreous ceramic coating on the waterwall panel in such a way that the coating is fused at the heated portions of the panel.
Typically the waterwall has first and second faces (i.e. of the tubes), and steps (a) and (b) are repeated so as to fuse the coating substantially continuously over substantially the entire first face of the waterwall panel. The induction heating in step (a) is preferably practiced at a frequency of greater than about 25 kHz, and may be practiced utilizing a portable compact transformer connected to a main power supply (and step (a) may be practiced at a distance of more than thirty feet from the power supply). Preferably step (a) is practiced by concentrating induction energy in the membrane portion of the waterwall, and step (b) is practiced before step (a).
Step (a) may be practiced by moving an induction coil having noses roughly approximating the contour of the waterwall panel over the panel. The method may comprise the further step of circulating a cooling fluid through the induction coil during the practice of step (a). In one embodiment step (b) is practiced by applying a nickel based alloy having a coating thickness of from 3-40 mils, while in another step (b) is practiced by painting or spraying a composition of low melting point frits and an inorganic binder, in slurry form, with a thickness of between 3-15 mils; and there is the further step of air drying the coating before the practice of step (a). Step (a) is typically practiced so as to heat the coating to a temperature of between about 1000-2200.degree. F.
Step (a) may be practiced by first passing a preheater coil assembly (which is typically a leading part of the fusion coil assembly), including a copper nose which extends down to the membrane without a flux concentrator, over the panel, and then passing a fusion coil assembly (which may be a trailing part of the preheater coil assembly), comprising a copper nose and magnetic flux concentrator which brings sufficient inductive energy to the membrane so that the coating on the membrane can be fused without overheating the coating on the tube, over the panel. It is also possible to fuse multiple tube-membrane configurations at once by increasing the size of the coil, and passing water through the tubes during the fusing process, as the induction coil provides heat to the coated surface faster than it can be extracted through the water.
According to another aspect of the present invention a method of fusing similar coatings on a complicated metal shape or convoluted metal surface, in general, is provided. The method comprises the steps of: (a) Applying a self-fusing alloy thermal spray coating or a vitreous ceramic coating on the complicated metal shape or convoluted metal surface so that the coating is fused at the heated portion thereof. And then (b) inductive heating at least a portion of the complicated metal shape or convoluted metal surface by induction at a frequency of greater than about 25 kHz to at least the liquidus temperature of the coating. The details of these steps, and any additional steps, are substantially as described above. In a preferred embodiment, step (b) is practiced by (i) first passing a preheater with at least one copper nose and without a flux concentrator so that it substantially conforms to the convoluted metal surface or complicated metal shape, and (ii) then passing over the surface or shape a fusion coil assembly having at least one copper nose with a magnetic flux concentrator substantially conforming to the convoluted metal surface or complicated metal shape so as to effect fusing; and wherein substeps (i) and (ii) are practiced using a unitary structure.
According to another aspect of the present invention an induction coil assembly is provided for practicing the methods as set forth above. The induction coil assembly comprises the following components: An electrically conductive material tubular combined electrical current conductor and conduit for circulating cooling fluid, and having a first closed end, and a second end connectable to a source of cooling fluid and a source of electricity. And, at least one electrically conductive material nose extending outwardly from the combined conductor and conduit and both conducting electricity and circulation cooling fluid, the nose extending substantially perpendicularly to the combined conductor and conduit, and the nose configured so as to effect induction heating of at least two differently configured portions of a complicated metal shape or convoluted metal surface.
The tubular combined conductor and conduit may be substantially quadrate (i.e. square or rectangle) in cross-section, or may be circular or oval in cross-section as well. A magnetic flux concentrator may be disposed on at least one of the at least one nose. The flux concentrator is connected to at least one of the nose and the combined conductor and conduit with a thermally conducive adhesive. The flux concentrator is preferably formed of magnetic particles and a dielectric material which serves as a binder and insulator which are pressed to form the shape thereof, or of a ferrite material. Preferably the combined conductor and conduit is copper and the nose is copper.
The coil is preferably used in combination with a transformer or capacitor station and a greater than about 25 kHz power supply electrically connected to the combined conductor and conduit. Usually a plurality of noses are provided, and the noses and combined conductor and conduit at a portion thereof approximate the surface configuration of a waterwall panel having a plurality of tubes connected by a plurality of membranes.
The coil assembly of the invention also typically comprises a plurality of coil positioners preferably spaced widely from each other and operatively connected to the combined conductor and conduit, for engaging the complicated metal shape or convoluted metal surface and guiding the at least one nose thereover so that the at least one fusing nose is properly positioned to effect induction heating of the shape or surface.
The tubular combined conductor and conduit may be in the form of a loop having a first portion which acts as a trailing portion in use, and a second portion which acts as a leading portion in use. The at least one nose for induction heating is on the first portion, and the assembly further comprises at least one preheating nose on the second portion. The at least one nose for induction heating typically includes a magnetic flux concentrator disposed thereon, and the at least one preheating nose comprises a solid block of copper brazed to the second portion, and devoid of a flux concentrator.
It is the primary object of the present invention to provide the efficient and effective fusing of thermal spray coatings and/or vitreous ceramic coatings on complicated metal shapes or convoluted surfaces, that need abrasion and corrosion protection, such as waterwall panels. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.